How to Cut Ceiling Joists for Attic Fans

Transfer the dimensions for the attic fan’s rough opening to a large piece of flat cardboard. The manufacturer’s installation instructions will give you the rough opening dimensions, and in some cases, you’ll find a paper template. Otherwise, just draw a square on the cardboard and cut it out.

Place the cardboard square on the ceiling where you want to install the attic fan, using a pencil to trace around the cardboard.

Fit a screw gun with a standard drill bit and drill four pilot holes through the ceiling drywall at the four corners of the square.

Go to the attic and locate the four holes. Remove any insulation and check for wires that may run through the ceiling joists. Standard ceiling joists sit 24 inches apart and at least one ceiling joist will probably fall within the square formed by the four corner holes.

Draw a line between the four holes on the attic side of the drywall for reference. The line will cross the ceiling joist to be cut in two places, at the top and at the bottom of the rough opening dimensions.

Measure 3 inches beyond the pencil mark on both ends of the joist to be cut, and make new marks. These are the joist-cutting marks.

Cut out the drywall along the pencil lines from the floor below with a utility knife to reveal the hole for the attic fan.

Cut the ceiling joist with a circular saw, from the attic. Use the two joist-cutting marks as guides. Since you’re cutting the joist farther back than the edge of the drywall hole, take care not to cut through the ceiling drywall.

Measure the distance between the two ceiling joists on either side of the joist you cut, and cut four pieces of dimensional lumber to match. If your ceiling joists are 24 inches apart, this measurement should be very close to 46 1/2 inches. Use lumber that is the same dimension as the ceiling joists. For example, if the ceiling joists are 2-by-8s, use 2-by-8 dimensional lumber.

Align one of the cut boards between the side joists and push it back until it meets the end of the joist you cut out. Use a nail gun and 8d nails to attach the new board to the side joists and to the end of the cut joist.

Position a second board right beside the first board and nail it to the first board and to the joists at both ends. This double board that spans the distance between the side joists is a header. The second board will be flush with the edge of the hole you cut in the drywall.

Install the other two boards to form another header on the other side of the rough opening in the same manner.

Measure the distance between the two headers and cut two boards to match. Nail the two boards between the headers you just constructed, flush with the edges of the drywall hole. Now, you have the rough opening in which to set the attic fan.